The bundling is a matter of membrane diameter and frequency. Bigger diameter result in stronger beam forming, however lower frequencies have less beam forming. You can see the effect very well in the directivity pattern of this 8" wide band speaker: https://www.visaton.de/sites/default/files/dd_product/B%2020...
I've a FAST/WAW with a 4" wideband (250Hz XO). As personal monitor speakers these work great with an amazing stage and "resolution" (the illusion of being able to pinpoint the exact spatial location of a single instrument).
//woah: Thanks you two (you&GP) for triggering me on this (not /s)! While grabbing a coffee I spent a few minutes thinking about beam forming in audio applications; and came up with the line array. Not so novel, I know, but I'm happy to have gotten there on my own.
What I love about Toole's book is it's basically the whole big picture of audio, completely sourced and grounded in real deal empirical research, but research that also worked to correlate objective measurements to subjective preferences.
All those internet arguments over the years about what matters and what doesn't, tubes vs transistors, class A vs class AB amps, you name it... Toole's book is the cliff notes of what's actually real. It's such a huge advantage vs when I first started reading this stuff in the 90s and it was a lot harder to find information that wasn't just someone's "golden ear" opinion imagined out of thin air.
Listeners prefer objectively accurate (ie, the input matches the output) audio reproduction, with some added emphasis on the bass frequencies. Because most of the sound we hear indoors is actually reflected, it is crucial that frequency response of the speakers remains constant as we move off axis.
This might seem blindingly obvious, especially the first part, but it's groundbreaking work as far as demystifying audio is concerned.
For so many decades, the snake oil salesmen dominated the hifi industry with bogus claims. They wanted us to think that building speakers was like crafting the perfect wine. But it's not. It's engineering. =)